Power Supply (so big you never notice the level going down) Tap Capacitor V Area represents capacitance, C Volume represents charge, Q Capacitor Model V

Metal Sheets Sheets of Dielectric (insulating material) C =  A / d For all insulators  >  0 For large capacitance we want a large area, A, and a small separation, d. The unit of Capacitance is the Farad (F). A 1F capacitor is huge – usually capacitors will be  F, although sometimes they will be mF or nF. How do you get a large capacitance in a small volume?

When you apply a potential difference, V, across a capacitor, you will store an amount of charge, Q on the capacitor, with a capacitance C. We find that… Q=VC Check the units – 1 Coulomb = 1 Volt x 1 Farad  This equation is on your data sheet, but not in this form.

Energy Stored in a Capacitor Q=VC Potential Difference Charge (Energy per unit charge) V Q Energy E = ½ Q V = ½ V 2 C = ½ Q 2 /C You must be able to argue: A graph of V against Q is a straight line through the origin V = Energy / Charge Area = (Energy/Charge)xCharge = Energy

Energy Stored in a Capacitor When circuit is fully charged, the power supply has provided Q coulombs at V volts, so has given out QV joules of energy. But only ½QV of energy is stored in the capacitor. Where has the rest gone? It has been lost as heat as the current passes through the wires. Optional Resistor Q V Energy out = QV Energy stored = ½QV

Energy Stored in a Capacitor Flashes in cameras store some energy in a capacitor, and then use this energy to fire the flash. But we know that doing this wastes at least half the energy in charging the capacitor. So why don’t we use the energy straight from the battery? (Hint: It has something to do with the internal resistance of the battery!) Battery Capacitor Flash Bulb Energy released quickly Current very high Battery’s terminal pd would be very low V =  - I r

PhET Simulation

Capacitor Charging circuit Optional Resistor Charging Circuit When circuit is fully charged: PD across capacitor = PD of power supply No current flows in the circuit Questions 3-5, 8, Page 273. Q30+31 P280